Power supply apparatus

ABSTRACT

A power supply apparatus includes a power supply section and a control section. The power supply section includes a first switch which performs switching of power-supply output, an inductor and a capacitor which form an LC circuit that resonates current flowing through the first switch, and a second switch which changes capacitance of the capacitor. The control section finds in advance a correspondence between a time ratio of a switching pulse of the first switch and a resonance frequency which matches an edge of a switching pulse having each time ratio, and controls switching of the second switch so as to resonate the LC circuit at a resonance frequency corresponding to a time ratio at operation time.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2013-196001, filed on Sep. 20,2013, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to a switching power supplyapparatus.

BACKGROUND

As semiconductor processes become minuter, in recent years the absolutevalue of power-supply voltage has become smaller and the requirements ofaccuracy with which voltage is set have become severer. In addition,with an increase in integration level, there is a tendency forpower-supply current to increase to several tens of amperes.

POL (Point Of Load) by which a power-supply circuit, such as anon-isolated step-down DC(Direct Current)-DC converter, is placed near adevice that is the load on a power supply is effective for systemshaving the above uses. By adopting the power supply architecture of thePOL, a loss caused by a drop in voltage is prevented or responsibilityis improved.

In order to accommodate fluctuations in current consumed by LSI (LargeScale Integration), it is desirable with the POL to make a switchingfrequency of a switching power supply as high as possible. However, if aswitching frequency is too high, great loss occurs in a switchingelement such as a FET (Field Effect Transistor).

A technique referred to as soft switching is known as a technology forpreventing such loss at switching time and stabilizing output voltage. Aresonant converter is widely known as typical soft switching.

For example, the following two techniques were proposed formerly as aresonant converter. One is to divide resonance capacitors and performswitching for changing a resonance frequency. By doing so, the resonancecapacitors have plural values according to switching frequencies. Theother is to change a resonance frequency by changing an inductancevalue.

Japanese Laid-open Patent Publication No. 01-248957

Japanese Laid-open Patent Publication No. 09-201044

When disturbance, such as fluctuations in load, occurs, control isexercised for stabilizing output voltage. If a resonance frequency isvariably set at this time, circuit structure is complex with the aboveconventional resonant converters.

In order to stabilize output voltage, control is exercised for switchinga resonance capacitor under the output condition or the input conditionunder which the value of a switching frequency is smaller than or equalto a limit value (see, for example, Japanese Laid-open PatentPublication No. 01-248957).

However, a switching frequency (switching timing) of a switching powersupply is sequentially made to match switching timing of the resonancecapacitor in an adaptive manner during operation. As a result, circuitstructure in a control system is complex.

In addition, a transformer-type inductor is used and a secondary side ofthe transformer is switched (see, for example, Japanese Laid-open PatentPublication No. 09-201044). As a result, great loss occurs on thesecondary side and a control circuit for preventing this loss is newlyplaced.

SUMMARY

According to an aspect, there is provided a power supply apparatus whichincludes a power supply section including a first switch which performsswitching of power-supply output, an inductor and a capacitor which forman LC circuit that resonates current flowing through the first switch,and a second switch which changes capacitance of the capacitor and acontrol section which finds in advance a correspondence between a timeratio of a switching pulse of the first switch and a resonance frequencythat matches an edge of a switching pulse having each time ratio andwhich controls switching of the second switch so as to resonate the LCcircuit at a resonance frequency corresponding to a time ratio atoperation time.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates an example of the structure of a power supplyapparatus;

FIG. 2 illustrates an example of the structure of a current resonanceconverter;

FIG. 3 indicates operational waveforms;

FIG. 4 indicates operational waveforms;

FIG. 5 illustrates an example of the structure of a power supplysection;

FIG. 6 illustrates an example of the structure of a control section;

FIG. 7 indicates the output waveform of a sawtooth wave generator;

FIG. 8 is a view for describing PWM signal generation operation;

FIG. 9 indicates an example the structure of a table stored in a memory;

FIG. 10 indicates an output voltage waveform;

FIG. 11 is a flow chart of the operation of the control section at thetime of being in training mode;

FIG. 12 is a view for describing the operation of the control section atthe time of being in the training mode;

FIG. 13 indicates a shift in waveform at switch switching time;

FIG. 14 indicates a shift in waveform at switch switching time;

FIG. 15 indicates a shift in waveform at switch switching time;

FIG. 16 indicates a shift in waveform at switch switching time;

FIG. 17 indicates a shift in waveform at switch switching time; and

FIG. 18 illustrates an example of the structure of the control section.

DESCRIPTION OF EMBODIMENTS

An embodiment will now be described with reference to the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout. FIG. 1 illustrates an example of the structure of a powersupply apparatus. A power supply apparatus 1 is a switching power supplyapparatus and includes a power supply section 10 and a control section20.

The power supply section 10 includes a direct-current power supply Vi, aswitch SW1 (first switch), a switch SWr (second switch), a switch SW2,inductors Lr and L1, and capacitors Cr and C1. The switch SW1 performsswitching of output from the direct-current power supply Vi on the basisof a switch control signal p1.

The inductor Lr and the capacitor Cr form an LC circuit which resonatescurrent flowing through the switch SW1. The switch SWr changes thecapacitance of the capacitor Cr on the basis of a switch control signalpr.

The inductor L1 and the capacitor C1 form a filter which smoothsdirect-current signal output. Furthermore, the switch SW2 is used forsupplying current to the inductor L1 on the basis of a switch controlsignal p2.

The control section 20 outputs the switch control signals p1 and pr tocontrol switching of the switches SW1 and SWr respectively. In addition,the control section 20 finds in advance the correspondence between atime ratio (ratio of switch-on time to switch-off time) of a switchingpulse of the switch SW1 and a resonance frequency which matches an edgeof a switching pulse having each time ratio.

The control section 20 then exercises control for switching the switchSWr on the basis of the switch control signal pr so as to resonate theLC circuit at a resonance frequency which corresponds to a time ratio atoperation time and which is found in advance.

As has been described, the power supply apparatus 1 finds in advance thecorrespondence between a time ratio of a switching pulse and a resonancefrequency which matches an edge of a switching pulse having each timeratio. Furthermore, the power supply apparatus 1 recognizes a resonancefrequency corresponding to a time ratio of a switching pulse atoperation time from among resonance frequencies found in advance, andcontrols the power supply section 10 on the basis of the recognizedresonance frequency.

As a result, the power supply apparatus 1 uses the resonance frequenciesfound in advance for exercising switching control so as to resonate theLC circuit. This makes it possible to stabilize output voltage by simplecircuit structure.

The circuit structure and operation of and problems with an ordinarycurrent resonance converter will now be described before the powersupply apparatus 1 according to the embodiment is described in detail.

A current resonance converter is known as a type of resonant converter.With the current resonance converter an LC circuit formed of an inductor(L) and a capacitor (C) is used for resonating a current waveform andmaking it a sine wave. The current resonance converter makes a switchingfrequency match a resonance frequency, makes the waveform of currentflowing through a switching element a resonance waveform, and turns onand off a switch at the time of the sine-wave current being zero. Bydoing so, switching loss is reduced.

FIG. 2 illustrates an example of the structure of a current resonanceconverter. FIG. 2 illustrates the (simplified) circuit structure of astep-down DC-DC converter. A current resonance converter 3 includes adirect-current power supply Vi, a switch SWa, inductors Lr1 and L1,capacitors Cr1 and C1, a diode D0, and a load resistor R0.

With the current resonance converter 3 an inductor and a capacitor areconnected to a switching power supply (corresponding to a PWM (PulseWidth Modulation) converter). That is to say, the resonance inductor Lr1is connected in series with the switch SWa included in a switching powersupply.

Furthermore, the resonance capacitor Cr1 is connected in parallel withthe diode D0 included in the switching power supply. The inductor L1 andthe capacitor C1 form a filter for smoothing direct-current signaloutput.

Each component is connected in the following way. A (+) terminal of thedirect-current power supply Vi is connected to one end of the switchSWa. The other end of the switch SWa is connected to one end of theinductor Lr1. The other end of the inductor Lr1 is connected to one endof the inductor L1, one end of the capacitor Cr1, and a cathode of thediode D0.

The other end of the inductor L1 is connected to one end of thecapacitor C1 and one end of the load resistor R0. A (−) terminal of thedirect-current power supply Vi is connected to the other end of thecapacitor Cr1, an anode of the diode D0, the other end of the capacitorC1, and the other end of the load resistor R0.

FIGS. 3 and 4 indicate operational waveforms. FIG. 3 indicates waveformsin a PWM converter operated by a square wave. FIG. 4 indicates waveformsin the current resonance converter 3 which is illustrated in FIG. 2 andwhich is operated by a sine wave.

FIG. 3 indicates the waveforms of a switching frequency S of a switchSWa included in a PWM converter, current Is1 which flows through theswitch SWa at the time of the switch SWa being in on and off states, andvoltage Vs1 applied to the switch SWa.

FIG. 4 indicates the waveforms of a switching frequency S of the switchSWa included in the current resonance converter 3, current Is2 whichflows through the switch SWa at the time of the switch SWa being in onand off states, and voltage Vs2 applied to the switch SWa. In each ofFIGS. 3 and 4, a vertical axis indicates voltage or current and ahorizontal axis indicates time.

In FIG. 3, when the switch SWa is in an on state, the current Is1 flowsthrough the switch SWa and voltage is not applied to the switch SWa.Furthermore, when the switch SWa is in an off state, current does notflow through the switch SWa and the voltage Vs1 is applied to the switchSWa.

In FIG. 4, when the switch SWa is turned on, the resonance inductor Lr1is excited by voltage inputted from the direct-current power supply Viand resonance current which flows through the resonance inductor Lr1,that is to say, the current Is2 which flows through the switch SWaincreases from zero like a sine wave.

After the current Is2 peaks, it decreases to zero like a sine wave. Inaddition, when the current Is2 which flows through the switch SWabecomes zero, the switch SWa is turned off. This switching operation isreferred to as zero-current switching.

With a hard switching type PWM converter like that illustrated in FIG.3, the waveforms of current which flows through a switch and voltageapplied to the switch are square waves. As a result, in a time zone inwhich a rise or a fall takes place at switching time, there is apossibility that current which flows through a switching elementoverlaps with voltage applied to the switching element. The occurrenceof such a phenomenon causes switching loss.

With a soft switching type current resonance converter like thatillustrated in FIG. 4, on the other hand, a resonance inductor and aresonance capacitor are located around a switch, current which flowsthrough the switch is made a sine wave by utilizing a resonancephenomenon, and zero-current switching is performed.

This reduces an overlap between voltage and current at switching timeand reduces switching loss. Furthermore, with soft switching parasiticelements (parasitic capacitance and parasitic inductance) in a circuitare incorporated in a resonance circuit. Accordingly, a soft switchingtype current resonance converter also has the advantage of being able toreduce a switching surge (high-frequency oscillation of voltage orcurrent which occurs at switching time) caused by these parasiticelements.

In order to stabilize output voltage, a switching power supply, such asa current resonance converter, accommodates a disturbance, such asfluctuations in input voltage or fluctuations in load, by exercisingcontrol so as to increase or decrease a time ratio of a switching pulse.

With the conventional current resonance converter 3, however, theinductance value of the resonance inductor Lr1 and the capacitance valueof the resonance capacitor Cr1 are fixed, so a resonance frequency isconstant.

Therefore, in order to stabilize output voltage, a time ratio of aswitching pulse may be changed for accommodating a disturbance. In thatcase, however, a switching frequency does not match the resonancefrequency. As a result, switching loss occurs and soft switching is notrealized.

Furthermore, in such a case, a time ratio of a switching pulse may beincreased or decreased by fluctuations in switching frequency in orderto make a switching frequency match the resonance frequency. By doingso, control is exercised for stabilizing output voltage.

However, if such control is exercised, a switching frequency changesaccording to a disturbance. As a result, a loop control band is limited(loop control band is set to about a tenth of a switching frequency orless in order to avert the influence of the switching frequency on acontrol system).

In addition, the frequency spectrum of noise produced by switching alsochanges according to a disturbance, so it is difficult to take a noisemeasure.

On the other hand, in order to reduce the influence of a ripple having aswitching frequency or make a response to a disturbance faster, it iseffective to adopt a multiphase system in which a plurality ofpower-supply circuits are arranged in parallel and in which switchingfrequencies are made to match.

However, if the above control is exercised for changing a switchingfrequency, a switching frequency of each power-supply circuit isindefinite. As a result, it is difficult to perform parallel operationof the plurality of power-supply circuits (multiphase system is notrealized).

Furthermore, the following technique is proposed. A switching frequencyis not changed. Control is exercised so as to variably set a resonancefrequency of a resonance circuit. By doing so, switching loss isreduced. As stated above, however, if the conventional resonantconverter in which a resonance frequency is variably set is adopted,circuit scale increases and circuit structure becomes more complex.

A power supply apparatus according to an embodiment is devised in viewof these problems. There is provided a power supply apparatus which hassimple circuit structure, which reduces switching loss, and whichstabilizes output voltage.

The power supply apparatus 1 according to an embodiment will now bedescribed in detail. First an example of the circuit structure of thepower supply section 10 illustrated in FIG. 1 will be described. FIG. 5illustrates an example of the structure of the power supply section. Astep-down DC-DC converter is taken as an example of the power supplysection 10.

A DC-DC converter 10 a includes a direct-current power supply Vi,switches SW1, SW2, and SWr, inductors Lr and L1, capacitors Cr1, Cr2,and C1, and a load resistor R0.

Each component is connected in the following way. A (+) terminal of thedirect-current power supply Vi is connected to one end of the switchSW1. The other end of the switch SW1 is connected to one end of theinductor Lr. The other end of the inductor Lr is connected to one end ofthe switch SWr, one end of the capacitor Cr2, one end of the switch SW2,and one end of the inductor L1. The other end of the switch SWr isconnected to one end of the capacitor Cr1.

The other end of the inductor L1 is connected to one end of thecapacitor C1 and one end of the load resistor R0. A (−) terminal of thedirect-current power supply Vi is connected to the other end of thecapacitor Cr1, the other end of the capacitor Cr2, the other end of theswitch SW2, the other end of the capacitor C1, and the other end of theload resistor R0.

Switch control signals p1, p2, and pr are supplied from the controlsection 20 to the switches SW1, SW2, and SWr respectively. Switching isperformed on the basis of the applied switch control signals p1, p2, andpr. The switches SW1, SW2, and SWr are switching elements and FETs, forexample, are used.

The inductor Lr is a resonance inductor and the two capacitors Cr1 andCr2 are resonance capacitors. Furthermore, the inductor L1 and thecapacitor C1 form a filter for smoothing direct-current signal output.

The switch SW1 is a main switch of a switching power supply. When theswitch SW1 is in an off state, the switch SW2 is in an on state andcurrent is supplied to the inductor L1 (when the switch SW1 is in an onstate, the switch SW2 is in an off state).

A diode may be used as the switch SW2. However, if a diode is used, theprobability that loss occurs is high. Therefore, it is desirable to usea transistor switch such as a FET.

The switch SWr is used for increasing or decreasing the capacitancevalue of the resonance capacitor Cr1 and changing a resonance frequency.A resonance frequency of LC resonance in the DC-DC converter 10 a isdetermined on the basis of the inductance value of the resonanceinductor Lr and a combined capacitance value of the resonance capacitorsCr1 and Cr2.

In this case, a combined capacitance value of the resonance capacitorsCr1 and Cr2 is changed by performing on-off switching of the switch SWron a time-division basis and equivalently increasing or decreasing thecapacitance value of the resonance capacitor Cr1.

A capacitance value which is one of the parameters by which a resonancefrequency is determined is variably set by controlling timing at whichthe switch SWr is switched. Accordingly, a resonance frequency(resonance cycle) of current flowing through the switch SW1 can be setto a desired value.

Switching is performed by the switch SWr on the capacitor Cr1, which isone of the capacitors Cr1 and Cr2 connected in parallel. By doing so, acapacitance value on a capacitor Cr1 side is increased or decreased.

This makes it possible to continuously change combined capacitance ofthe capacitors Cr1 and Cr2 and continuously follow a time ratio of aswitching pulse (when a switching frequency changes, a resonancefrequency can continuously be made to match the switching frequency).

Furthermore, if the resonance capacitors Cr1 and Cr2 are connected inparallel, the capacitance of the resonance capacitor Cr1 is made smallerthan that of the resonance capacitor Cr2. The reason for this is asfollows. Switching of input to a capacitor whose capacitance is high isa factor in the occurrence of noise or the like. By connecting the abovecapacitors Cr1 and Cr2 in parallel, a factor in the occurrence of noiseat switching time is removed.

An example of the structure of the control section 20 illustrated inFIG. 1 will now be described. FIG. 6 illustrates an example of thestructure of the control section. A control section 20-1 is a blockwhich controls the whole of the power supply apparatus 1, and controlsswitching of each of the switches SW1, SW2, and SWr as one function.

The control section 20-1 includes an amplifier 21, a reference voltagegenerator 22, a loop compensation circuit 23, a sawtooth wave generator24, A/D (Analog/Digital) converters 25 a and 25 b, a memory 26, anoperation processor 27, a D/A (Digital/Analog) converter 28, comparators29 a and 29 b, and drivers Dr1 through Dr3.

The amplifier 21 detects an error signal which is the differentialbetween output voltage Vout outputted from the DC-DC converter 10 a andreference voltage Ref outputted from the reference voltage generator 22,and amplifies and outputs the error signal.

The loop compensation circuit 23 exercises loop compensation control inthe power supply apparatus 1 on the basis of an error signal outputtedfrom the amplifier 21. Roughly speaking, output from the DC-DC converter10 a, error detection by the amplifier 21, loop compensation by the loopcompensation circuit 23, control of the switches included in the DC-DCconverter 10 a, and output from the DC-DC converter 10 a form a controlsystem loop in the power supply apparatus 1. That is to say, a negativefeedback loop for keeping output voltage Vout constant is formed.

In this case, when a signal makes a round of the loop and its phase isinverted, negative feedback is realized. For example, however, if thephase of a signal which has made a round of the loop is the same as thatof an original signal (in-phase) and the gain is 1 (0 dB), oscillationconditions are met and oscillation takes place.

Therefore, on the basis of an error signal inputted, the loopcompensation circuit 23 generates a loop compensation signal (voltagesignal) for controlling the switches SW1 and SW2 so as to make the powersupply apparatus 1 deviate from the oscillation conditions as far aspossible.

The sawtooth wave generator 24 generates a sawtooth wave with determinedwidth in a determined cycle. The comparator 29 a compares a loopcompensation signal and a sawtooth wave and generates a PWM signal withdetermined pulse width.

On the basis of a PWM signal, the driver Dr1 outputs a switch controlsignal p1 for controlling switching of the switch SW1. Furthermore, onthe basis of a PWM signal, the driver Dr2 outputs a switch controlsignal p2 for controlling switching of the switch SW2.

In this case, the driver Dr2 outputs a switch control signal p2 whosephase is inverted so that the on and off states of the switch SW2 willcorrespond to the off and on states, respectively, of the switch SW1.

On the other hand, the A/D converter 25 a converts an analog loopcompensation signal to a digital signal. The memory 26 stores thecorrespondence between a time ratio of a switching pulse of the switchSW1 or SW2 and a resonance frequency which matches an edge of aswitching pulse having each time ratio.

The operation processor 27 is a CPU (Central Processing Unit), a digitaloperation circuit, or the like. When the operation processor 27 receivesa digital value of a loop compensation signal, the operation processor27 recognizes an output voltage value of the driver Dr3 and generates adriving signal, on the basis of contents registered in the memory 26.

The D/A converter 28 converts a digital driving signal to an analogdriving signal. The comparator 29 b compares an analog driving signaland a sawtooth wave and generates a PWM signal with determined pulsewidth. On the basis of a PWM signal, the driver Dr3 outputs a switchcontrol signal pr for controlling switching of the switch SWr.

On the other hand, the A/D converter 25 b converts current Isw flowingthrough the switch SW1 (current Isw flows at a falling edge of aswitching pulse of the switch SW1) to a digital signal. The operationprocessor 27 generates data to be registered in the memory 26 intraining mode described later on the basis of a digital value of currentIsw flowing through the switch SW1.

PWM signal generation operation will now be described. FIG. 7 indicatesthe output waveform of the sawtooth wave generator. In FIG. 7, avertical axis indicates voltage and a horizontal axis indicates time. Asawtooth wave d2 having a shape illustrated in FIG. 7 is outputted fromthe sawtooth wave generator 24. For the sake of simplicity the minimumvoltage and maximum voltage of the sawtooth wave d2 are 0 and 1 Vrespectively.

FIG. 8 is a view for describing PWM signal generation operation. On eachgraph in FIG. 8, a vertical axis indicates voltage and a horizontal axisindicates time. PWM (Pulse Width Modulation) is a process for convertingthe magnitude of a signal wave to the magnitude of pulse width, and isrealized by, for example, a circuit illustrated in FIG. 8.

A voltage signal d1 (loop compensation signal outputted from the loopcompensation circuit 23, for example) is inputted to a (+) terminal of acomparator 29 and the sawtooth wave d2 (signal generated by the sawtoothwave generator 24) is inputted to a (−) terminal of the comparator 29.

When voltage inputted to the (+) terminal is higher than voltageinputted to the (−) terminal, the comparator 29 outputs a H level.Accordingly, when the level of the voltage signal d1 is higher than thatof the sawtooth wave d2 in this case, output from the comparator 29 isat a H level and a pulse signal d3 (PWM signal) is outputted from thecomparator 29.

When the voltage signal d1 is at a low level, the pulse width of thepulse signal d3 is small. When the voltage signal d1 is at a high level,the pulse width of the pulse signal d3 is large. This means that pulsewidth modulation is performed.

A table stored in the memory 26 will now be described. FIG. 9 indicatesan example the structure of a table stored in the memory. A table T1includes Loop Compensation Signal Value and Driver Output Value items.

A loop compensation signal value is obtained by digitizing output fromthe loop compensation circuit 23, and corresponds to a duty of a drivingwaveform of the switch SW1 or SW2. Furthermore, a driver output value isa digital value corresponding to an output voltage value of the driverDr3, and corresponds to a duty of a driving waveform of the resonanceswitch SWr (each value indicated in FIG. 9 is an example and operationis not verified).

When the operation processor 27 receives a digital voltage value of aloop compensation signal, the operation processor 27 refers to valuesregistered in the table T1 held in the memory 26, and acquires an outputvoltage value of the driver Dr3. This means that a duty of a drivingwaveform of the resonance switch SWr corresponding to a duty of adriving waveform of the switch SW1 or SW2 is acquired.

FIG. 10 indicates an output voltage waveform based on the table T1. FIG.10 indicates the output voltage waveform of the driver Dr3. In FIG. 10,a vertical axis indicates output voltage of the driver Dr3 and ahorizontal axis indicates a voltage value of a loop compensation signal.

As has been described, the control section 20 registers in a table atime ratio of a driving waveform of the switch SW1 (and the switch SW2)as information regarding a time ratio of a switching pulse and a timeratio of a driving waveform of the resonance switch SWr as informationregarding a resonance frequency and holds the table in the memory 26.

As a result, a resonance frequency (duty of a driving waveform of theswitch SWr) corresponding to a switching frequency of the switch SW1 orSW2 is easily acquired from the memory 26 in an instant during systemoperation. This makes it possible to control a switching power supplywithout using a complex circuit.

The operation of measuring in the training mode the relationship betweenswitching timing of a switching power supply and a resonance frequencywill now be described. The control section 20 operates in the trainingmode in which it performs switching of the resonance switch SWr,regulates a resonance frequency, and acquires data.

FIG. 11 is a flow chart of the operation of the control section at thetime of being in the training mode. Furthermore, FIG. 12 is a view fordescribing the operation of the control section at the time of being inthe training mode. In FIG. 12, the flow indicated in FIG. 11 isintelligibly described by the use of waveforms.

(Step S0) The control section 20 outputs a switch control signal p1 tothe switch SW1 and outputs a switch control signal p2 to the switch SW2.At this time the pulse width of the switch control signal p1 (or theswitch control signal p2) is made narrowest in a used range.

(Step S1, State a) The control section 20 outputs a switch controlsignal pr to the switch SWr. At this time (at the beginning of training)the control section 20 controls the switch SWr so as to make a fallingedge (on-off edge) of the switch control signal pr equal to a risingedge (off-on edge) of the switch control signal p1 in timing.

(Step S2, State b) The control section 20 monitors a value of currentflowing through the switch SW1 at a falling edge (on-off edge) of theswitch control signal p1 and sample-and-holds the current value.

(Step S3) The control section 20 determines whether or not the absolutevalue of the current value which it sample-and-holds is smaller than areference value set in advance. If the absolute value of the currentvalue is smaller than or equal to the reference value, then the controlsection 20 proceeds to step S5. If the absolute value of the currentvalue is greater than the reference value, then the control section 20proceeds to step S4.

(Step S4, State c) The control section 20 shifts the falling edge(on-off edge) of the switch control signal pr by time Lt. The controlsection 20 then returns to step S2.

(Step S5, State d) The control section 20 registers in the memory 26 thewidth of a pulse of the switch SW1 and the width of a pulse of theswitch SWr found from the position of the falling edge (on→off edge) ofthe switch control signal pr.

The width of a pulse of the switch SW1 is a duty of a switching pulse ofthe switch SW1 and the width of a pulse of the switch SWr is a duty of aswitching pulse of the switch SWr.

(Step S6) The control section 20 determines whether or not the pulsewidth of the switch control signal p1 is greater than a maximum setvalue. If the pulse width of the switch control signal p1 is greaterthan the maximum set value, then the control section 20 ends theoperation. If the pulse width of the switch control signal p1 is notgreater than the maximum set value, then the control section 20 proceedsto step S7.

(Step S7) The control section 20 increases the pulse width of the switchcontrol signal p1 by time Lk. The control section 20 then returns tostep S1.

In steps S6 and S7, the control section 20 increases the width of aswitching pulse of the switch SW1 little by little to the maximum setvalue and finds a duty of a switching pulse of the switch SWr by which aresonance frequency corresponding to a duty of a switching pulse afteran increase is realized. The control section 20 measures and acquires inadvance plural data values corresponding to a disturbance, such asfluctuations in load, in this way.

In the above training sequence, the control section 20 monitors currentflowing through the switch SW1 at a current resonance frequency in atime zone in which there is a falling edge of a switching pulse of theswitch SW1.

If a monitored value is smaller than or equal to the reference value,then the control section 20 determines that zero-current switching isrealized at the current resonance frequency, and holds informationregarding the current resonance frequency. If a monitored value isgreater than the reference value, then the control section 20 determinesthat zero-current switching is not realized at the current resonancefrequency, performs switching for shifting a falling edge of a switchingpulse of the switch SWr by determined width, and changes the currentresonance frequency.

The above training sequence makes it possible to continuously changesthe resonance cycle of a current waveform of the switch SW1 and acquiredata (duty of a driving waveform of the switch SWr) by which a zeroswitching point is accurately set according to various disturbances suchas fluctuations in load.

After the power supply apparatus 1 shifts to steady-state operationmode, the control section 20 observes the difference between outputvoltage and reference voltage, an internal state, and the like and findsa time ratio of a switching pulse of the switch SW1 by performing anoperation and using an analog filter. This is the same with aconventional power supply.

After the control section 20 determines a time ratio of a switchingpulse of the switch SW1, the control section 20 reads out from thememory 26 a duty (switch-on time) of a switching pulse of the switch SWrby which a resonance frequency corresponding to the time ratio can berealized, and controls the switch SWr.

In this case, it is assumed that the capacitance values of the resonancecapacitors Cr1 and Cr2 are Ca and Cb respectively. When the switch SW1changes from an off state to an on state, the control section 20 turnson the switch SWr and makes a capacitance value of the resonancecapacitors Cr1 and Cr2 (Ca+Cb).

The control section 20 then turns off the switch SWr in a period forwhich the switch SW1 is in an on state. From this moment onward only thecapacitance value Cb of the capacitor Cr2 is used as a capacitance valueof a resonance capacitor.

By setting a capacitance value of a resonance capacitor in this way on atime-division basis, a resonance frequency is continuously changed. Inaddition, a deviation between an edge of a switching signal and an edgeof resonance current in soft switching is efficiently reduced.

Simulation results of a shift in waveform in switch control will now bedescribed. Each of FIGS. 13 through 17 indicates a shift in waveform atswitch switching time. In FIGS. 13 through 17, a vertical axis of eachof graphs g1 and g3 indicates voltage, a vertical axis of a graph g2indicates current, a vertical axis of a graph g4 indicates powerdensity, and a horizontal axis of each of the graphs g1 through g4indicates time.

It is assumed that the timing of switching of the switch SWr indicatedin FIG. 13 is considered as a start. FIG. 14 indicates a state in whichthe timing of switching of the switch SWr is delayed for 20 ns from thetiming indicated in FIG. 13. FIG. 15 indicates a state in which thetiming of switching of the switch SWr is delayed for 40 ns from thetiming indicated in FIG. 13. FIG. 16 indicates a state in which thetiming of switching of the switch SWr is delayed for 80 ns from thetiming indicated in FIG. 13. FIG. 17 indicates a state in which thetiming of switching of the switch SWr is delayed for 100 ns from thetiming indicated in FIG. 13.

Furthermore, a signal a1 on the graph g1 corresponds to a switch controlsignal p1 used for controlling switching of the switch SW1, and a signala2 on the graph g1 corresponds to a switch control signal pr used forcontrolling switching of the switch SWr (when each signal is at a Hlevel, each switch is in an on state).

The graph g2 indicates current flowing through the switch SW1. The graphg3 indicates voltage across the switch SW1. The graph g4 indicatesswitching loss which occurs in the switch SW1.

At the timing of the switching of the switch SWr indicated in FIG. 15 ofFIGS. 13 through 17, a resonance cycle matches time at which the switchSW1 turns on. As a result, the amount of switching loss which occurs inthe switch SW1 is smallest (approximately zero).

As can be seen from FIGS. 13 through 17, as the timing of the switchingof the switch SWr shifts, the cycle of current resonance changes. Inaddition, if the cycle of current resonance matches time at which theswitch SW1 turns on (FIG. 15), loss which occurs at the time ofswitching of the switch SW1 is smallest.

With the waveforms indicated in FIGS. 13 through 17, ideal elementmodels are used for making circuit operation simple. In a real circuit,however, parasitic inductance or parasitic capacitance of a switchingelement or a board causes an overshoot or a ringing in a waveform.

A modification of the control section 20 will now be described. FIG. 18illustrates an example of the structure of the control section. Acontrol section 20-2 includes an A/D converter 31, a reference voltagegenerator 32, a memory 33, a DSP (Digital Signal Processor) 34, PWMgeneration controllers 35 a through 35 c, and drivers Dr1 through Dr3.

The A/D converter 31 converts analog direct-current voltage Voutgenerated by the DC-DC converter 10 a, current Isw flowing through theswitch SW1, and analog reference voltage Ref outputted from thereference voltage generator 32 to digital signals.

The memory 33 holds the table T1 indicated in FIG. 9.

The DSP 34 performs digital operations for performing processesincluding error detection by comparing voltage outputted from the powersupply apparatus and the reference voltage, loop compensation control,and generation of a pulse of the switch SWr for generating a resonancecycle most suitable for a duty corresponding to a loop compensationsignal.

Each of the PWM generation controllers 35 a through 35 c generates a PWMsignal on the basis of a signal outputted from the DSP 34. The driversDr1 through Dr3 exercise drive control which is the same as thatdescribed in FIG. 6 to drive the switches SW1, SW2, and SWrrespectively.

As has been described in the foregoing, with the power supply apparatus1 according to the embodiment a PWM waveform (switching pulse of theswitch SW1) for soft switching is monitored and a capacitor switchingpulse (driving pulse of the switch SWr) by which the timing of an edgeof the PWM waveform matches the timing of an edge of a current resonancewaveform is generated in the control section 20.

In this case, the control section 20 also finds in advance thecorrespondence between a time ratio of a switching pulse and a resonancefrequency which matches an edge of a switching pulse having each timeratio. The control section 20 sets a resonance frequency correspondingto a time ratio of a switching pulse at operation time from amongresonance frequencies found in advance.

This reduces circuit scale in a control system and output voltage isstabilized by simple circuit structure. Furthermore, timing control forsoft switching and timing control of a resonance frequency can beexercised independently of each other, so a power supply is controlledmore flexibly.

In addition, with the POL in which there are a plurality of switchingpower supplies, for example, the timing of resonance capacitor switchingis controlled individually for plural switching frequencies of theplurality of switching power supplies. As a result, a plurality ofresonant converters are easily controlled in block by one controlsystem.

The embodiment has been described. However, a component included in eachsection indicated in the embodiment may be replaced with anothercomponent having the same function. In addition, any other component orprocess may be added.

According to the disclosed power supply apparatus, output voltage isstabilized by simple circuit structure.

All examples and conditional language provided herein are intended forthe pedagogical purposes of aiding the reader in understanding theinvention and the concepts contributed by the inventor to further theart, and are not to be construed as limitations to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although one or more embodiments of thepresent invention have been described in detail, it should be understoodthat various changes, substitutions, and alterations could be madehereto without departing from the spirit and scope of the invention.

What is claimed is:
 1. A power supply apparatus comprising: a powersupply section including: a first switch which performs switching ofpower-supply output; an inductor and a capacitor which form an LCcircuit that resonates current flowing through the first switch; and asecond switch which changes capacitance of the capacitor; and a controlsection which finds in advance a correspondence between a time ratio ofa switching pulse of the first switch and a resonance frequency thatmatches an edge of a switching pulse having each time ratio and whichcontrols switching of the second switch so as to resonate the LC circuitat a resonance frequency corresponding to a time ratio at operationtime.
 2. The power supply apparatus according to claim 1, wherein: thecontrol section operates in training mode in which the control sectionperforms switching of the second switch to regulate a resonancefrequency; in the training mode the control section monitors currentflowing through the first switch at a current resonance frequency in atime zone in which there is a falling edge of a switching pulse of thefirst switch; when a monitored value is smaller than or equal to areference value, the control section determines that zero-currentswitching is realized at the current resonance frequency, and holdsinformation regarding the current resonance frequency; and when themonitored value is greater than the reference value, the control sectiondetermines that zero-current switching is not realized at the currentresonance frequency, and changes the current resonance frequency byperforming switching for shifting a falling edge of a switching pulse ofthe second switch by determined width.
 3. The power supply apparatusaccording to claim 1, wherein the control section registers in a table atime ratio of a driving waveform of the first switch as informationregarding a time ratio of a switching pulse and a time ratio of adriving waveform of the second switch as information regarding aresonance frequency and holds the table in a memory.
 4. The power supplyapparatus according to claim 1, wherein: the LC circuit includes a firstcapacitor and a second capacitor; capacitance of the second capacitor isfixed; capacitance of the first capacitor is increased or decreased bythe second switch; and the capacitance of the first capacitor is lowerthan the capacitance of the second capacitor.
 5. The power supplyapparatus according to claim 4, wherein: when the first switch changesfrom an off state to an on state, the control section turns on thesecond switch and uses combined capacitance of the first capacitor andthe second capacitor as a capacitance value of a resonance capacitor ofthe LC circuit; while the first switch is in the on state, the controlsection turns off the second switch and uses the capacitance of thefirst capacitor as the capacitance value of the resonance capacitor ofthe LC circuit; and the control section performs switching of thecapacitance value of the resonance capacitor on a time-division basis.